Although parenterally administered capsular polysaccharide vaccine is highly effective in preventing pneumococcal pneumonia in adults, it appears less effective in preventing otitis media in infants. Thus there is a need to explore alternate approaches to prevention of this type of ear disease. We have selected ts mutants of S. pneumoniae which grow vigorously at 32 degrees C, but which do not replicate efficiently at 38 degrees C. Mutants of this type should replicate in the nasopharynx and stimulate local antibody, but should not produce disease in the middle ear or lower respiratory tract because of their failure to grow to high titer at the restrictive temperature of these areas. Ts mutants of type 1 S. pneumoniae were selected following exposure of virulent (ts+) organisms to nitrosoquanidine. Each mutant resembled the ts+ parent in bacteriologic and serologic properties, but was restricted in capacity to form colonies at 38 degrees C. When inoculated I.P. into mice, 11 of 13 mutants were attenuated and induced homologous resistance. Three mutants were also attenuated and immunogenic following administration I.P. to hamsters. Attenuation in hamsters may be related to decreased growth and survival of ts organisms at body temperature. Two mutants were completely attenuated for hamsters when administered intranasally and induced resistance to subsequent challenge with ts+ organisms by the same route.